Waste disposal has become increasingly problematic in recent years, and plastic packaging is perceived to be a major contributor to the problem. The availability of landfill space is decreasing, and international restrictions on the disposal of plastics at sea will be imposed in 1994. According to some estimates, conventional plastics require hundreds of years to decompose. In response to these concerns, there has been much activity in recent years in the development of biodegradable packaging materials. Polymers such as chitosan, polyvinyl alcohol (PVOH) and polyhydroxybutyrate-co-hydroxyvalerate have been investigated for these applications.
Many attempts have been made to produce biodegradable films from petroleum and cellulose derived materials. None has been completely successful because either they are too costly or decompose too slowly for most applications. Starch is probably the most abundant, low-cost, biodegradable polymer available, and its use in plastic film production could greatly reduce the demand for petrochemicals and the harmful impact on the environment caused by discarding nonbiodegradable plastic films.
As described in U.S. Pat. No. 4,673,438 to Wittwer et al., and U.S. Pat. Nos. 4,133,784, 4,337,181, and 4,454,268 to Otey et al., many starch-based biodegradable formulations use starch that has first been "destructurized" or "gelatinized". Destructurization or gelatinization is accomplished by heating the raw starch granules in the presence of water under elevated pressure. This treatment produces a disordering of the starch granules and allows the starch to be more effectively blended in conventional processing and production steps. However, destructurizing or gelatinizing the starch requires an additional pass through an extruder which increases the time and cost required to make starch-based biodegradable articles.
Starch must be combined with other materials (copolymers) in order to produce a satisfactory extruded film because extrusion of starch alone produces a brittle, water-sensitive foam. Addition of polyethylene or polypropylene are known to add water stability, elasticity, and toughness to processed starch-filled films. Unfortunately, polyethylene and polypropylene are compounds which have been shown not to be biodegradable. As a result, only the starch portion of the composite film biodegrades while the remaining copolymers remain intact. In U.S. Pat. No. 5,087,650 to Willett et al., for example, olefins such as ethylene and propylene are copolymerized with comonomers such as methyl acrylate, ethyl acrylate, and hexyl acrylate to produce a graft copolymer which is then combined with starch. The graft copolymer is not biodegradable, and thus the final product is actually only partially biodegradable due to decomposition of the starch component.
Conventional preparation of starch-based films, such as that described in U.S. Pat. No. 4,337,181 to Otey et al., requires a multiple step process in which starch is gelatinized, combined with pellets of nonbiodegradable copolymer, and processed into a film or other shaped article using a single screw extrusion process. In the process described in U.S. Pat. No. 5,087,650 to Willett, a starch-based "masterbatch" is formed by combining starch with copolymers, chopping the masterbatch extrudate into pellets, then blending the masterbatch pellets with polyolefins such as polyethylene or polypropylene which then can be injection molded or blown into film. Due to the fixed proportion of starch and copolymer in the pellets utilized in the prior art, ratios of starch and copolymer are not easily tailored to specific product requirements. Therefore, production of biodegradable starch-based products with different ratios of starch and copolymer are not easily manufactured. Furthermore, these conventional techniques of producing a starch-based film require additional steps that take extra time and add to the cost of the final articles.